1. Field of the Invention
The present invention relates to an automobile sensor package for detecting the magnitude of a tensile force in a seat belt used in a car seat, and in particular to a sensor package that can detect the magnitude of tension in a seat belt and provide an electrical signal that is representative of the magnitude of the tensile force.
2. Description of the Related Art
Various devices are well known for their ability to measure force, pressure, acceleration, temperature, position, etc. by using a sensing structure combined with signal processing electronics. One general type of sensor or transducer for such applications is a resistive strain gauge sensor in which force or pressure is sensed or measured based on strain placed on the resistors. Resistive strain gauges function by exhibiting changes in resistance proportional to strain which causes dimensional changes of the resistor.
Many types of strain gauge sensors have been designed and made commercially available. Various strain gauge sensors have proven to be generally satisfactory; however, these have tended to be rather expensive and not suitable in certain applications such as sensing the presence of an occupant in an automobile seat. A sensor suitable for such an application must be compact, robust, impervious to shock and vibration and yet inexpensive. In this regard, a sensor which has promise is described in U.S. Pat. No. 5,661,245 to Svoboda et al, issued Aug. 26, 1997, hereby herein incorporated by reference.
Automobile seats can use sensors to activate air bags, which would be deployed during an accident. Injury to infants or small children from air bag deployment with excessive force is a current industry problem. A weight sensor in the seat can be used to control the deployment force during air bag activation. Unfortunately, however, there are several problems with detecting seat occupant weight. For example, when a seated occupant puts on a seat belt, the force of cinching down the seat belt on the occupant can cause a seat weight sensor to have false and erroneous readings. For another example, if a child""s car seat is cinched down tightly in the car seat, it can appear to the weight sensor that a heavy person is in the seat, which is the wrong reading.
An example of a child seat sensing system is schematically depicted at FIG. 1, wherein a child seat 10 is placed upon a front passenger seat 12 and held thereto by a tightened seat belt 14. In this regard by way of example, the seat belt has an outboard portion 14o and an inboard portion 14i which are mutually coupled by a buckle 14b. The inboard portion 14i has a fixed length and is connected via an inboard anchor 16 to a vehicle component, such as for example a floor frame member. The outboard portion 14o is associated with, for example, an outboard anchor 18 which is also connected with a vehicle component. A shoulder belt 26 is associated with the outboard portion 14o, and is, for example, connected to a retractor assembly 22, which is, in turn, connected to a vehicle component. A weight sensor 20 provides a signal to the controller. When a crash is sensed by the crash sensor, the controller manages inflation of the air bag 24 via an air bag actuation circuit. The foregoing sensing scheme is described in detail in U.S. Pat. No. 5,454,591 to Mazur et al, issued Oct. 3, 1995, hereby herein incorporated by reference.
As represented schematically by FIG. 2, a seat belt tension sensor (BTS), which in general is used to measure the seat belt webbing tension, can be packaged in a number of locations. For example, a BTS could be packaged adjacent the outboard anchor 18, adjacent the inboard anchor 16, or somewhere at the buckle 14b. Each location has advantages and disadvantages. The BTS is required to compensate the weight sensing system such that federal government regulation FMVSS 208 may be met. This new regulation requires auto manufacturers to provide an automatic shut off of the passenger side air bag. The weight sensor may make vacover judgments under normal seating conditions. However, when a child seat is placed onto the vehicle seat and the seat belt webbing is used to cinch the child seat in place, a weight error is introduced into the sensing system. By gauging the webbing tension, the weight sensor can correct for the induced error due to the belt webbing so as to ensure the controller correctly determines whether to actuate, or whether to actuate and regulate the inflation force of, an air bag.
It can be seen from FIG. 2 that the seat belt 14 forms a load loop, the origin of which can be considered to be located at the buckle 14b where the latch thereof engages a tongue 14t connected to the end of the outboard portion 14o of the seat belt 14. This area in or near the seat belt buckle is a first possible BTS location. However, a BTS could be placed adjacent the outboard anchor 18. In this case, the rather long length of the outboard portion 14o of the seat belt 14 presents the possibility for a large amount of friction to be present between the buckle and the outboard anchor.
With the foregoing having been said, the aforementioned advantages and disadvantages of BTS location are as follows. With regard to BTS placement adjacent the outboard retractor, advantages include limited cross axis loading variation (discussed hereinbelow), greater amount of room for packaging, and ability to be covered so as to eliminate surface requirements and avoidance of splash and debris contamination; while disadvantages include greater amount of friction from D ring and occupant body friction sources, long distance from critical contact force location (tongue to latch contact location), and specific mounting requirements due to retractor mounting considerations. With regard to BTS placement inboard adjacent the buckle or in the buckle, advantages include the sensor being located close to the contact force of the tongue to latch with a consequent lowest possible system friction therebetween, possibility for integration into the same wiring harness as the buckle switch (one dual sensor assembly), the BTS could replace buckle switch if properly designed, and a low deflection is required due to close contact force proximity (which is a key consideration for reducing hysteresis and repeatability errors); while disadvantages include a high cross axis loading being required due to buckle head flexibility, packaging considerations must include prevention of possible contamination due to socover particles and liquid spills, and packaging may be more difficult due to small size requirement for the buckle area (requiring miniaturization).
Another consideration with respect to BTS placement is cross axis loading. In this regard, it should be appreciated that due to the fixed mounting in an outboard anchor based BTS, there would be limited cross axis loading, but that a buckle based BTS would have a worst case operating cross axis loading. This can be understood from FIGS. 3A and 3B.
FIG. 3A shows the details of the potential for cross axis loading at the buckle, wherein it is assumed that the buckle 14b is located within a cross axis load motion cone 30. The cone 30 is used to define a potential buckle position within or on the cone surface. In the example of FIG. 30A, the cone 30 begins with a cross-section of 24 mm and increases to a cross-section of 100 mm. It should be appreciated that the geometry of the cone 30 should be specifically defined by the seat belt supplier in combination with the seat supplier.
FIG. 3B depicts schematically the nature of the forces involved in cross axis loading. The actual load required to achieve the motion depicted in FIG. 3A may be quite large. The cross axis load motion cone 30 is defined by a forward direction loading force X, an inboard direction loading force Y (which is perpendicular to force X) and a twist moment xc2x1Mxy in the X-Y plane. Table 1 defines the range of cross axis loading motion including buckle twist.
Accordingly a need in the art continues to exist for a reliable, low cost, simple and robust seat belt tension sensor that can be used to input additional seat occupant information to an airbag controller to control airbag deployment, and which is insensitive to cross axis loading.
The present invention is a seat belt tension sensor package which is reliable, low cost, simple, robust, and usable to input additional seat occupant information to an airbag controller to control airbag deployment, and which is insensitive to cross axis loading.
The seat belt tension sensor package according to the present invention includes a first housing member, a second housing member internally interfaced with the first housing member, a suspension system for frictionlessly suspending the first housing member with respect to the second housing member, a pressure sensor mounted to one of the first and second housings, and a biasing spring mounted to the other of the first and second housings in axial abutment with the pressure sensor.
A seat belt is connected to the first and second housings, wherein tensile force of the seat belt is registered at the pressure sensor. The first and second housings are permitted a predetermined small axial movement, the axial movement being defined between a first relative position and a second relative position. The first relative position is defined by a zero tensile force axially applied to the first and second housings, and the second relative position is defined by a predetermined tensile force axially applied to the first and second housings. The axial movement occurs without scovering friction via the suspension system. Between the first and second relative positions, the axial tensile force applied to the first and second housings is taken by the suspension system and the biasing spring. However, at the second position, all increases in axial tensile force are taken by a mechanical abutment between the first and second housings.
The preferred suspension system utilizes a plurality of leaf springs which are freely flexible in the axial direction but quite inflexible in directions perpendicular thereto. As a result, the suspension system is very resistant to cross axis loading.
Accordingly, it is an object of the present invention to provide a seat belt tension sensor package which is reliable, low cost, simple and robust, and which is usable, for example, to input additional seat occupant information to an airbag controller to control airbag deployment.
It is an additional object of the present invention to provide a seat belt tension sensor package as aforedescribed which is insensitive to cross axis loading.
These, and additional and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.